T24d: Agent, previews

- BrowserPreview: the address with a lock and a spinner while it loads, the
  host's frame shown, and the frames so far along the foot as Tab stops;
  ScreenshotStream is that strip
- ComputerUseViewer: the host's frame, a ring where the agent points that
  rings again at each new point, and what it does there
- LivePreview: the host's page under its address, with reload
- ArtifactPanel: title, versions through chat::BranchNavigator, Preview and
  Source when it has both, copy and download; it fails loud with neither
- gallery: frames cut from the gallery's own pages; one asset! macro in
  main.rs replaces three copies

src/agent/mod.rs

@@ -1,11 +1,15 @@
+mod artifact;
 mod changes;
 mod permission;
+mod previews;
 mod progress;
 #[cfg(all(test, feature = "test-support"))]
 mod tests;
 mod tools;
 
+pub use artifact::ArtifactPanel;
 pub use changes::{ChangeState, FileChange, FileChangeCard, MultiFileDiffReview};
 pub use permission::{Permission, PermissionPrompt, ToolApprovalDialog};
+pub use previews::{BrowserPreview, ComputerUseViewer, LivePreview};
 pub use progress::{AgentPlan, AgentProgress, AgentState, AgentStatus, AgentStep, AgentStepList};
 pub use tools::{ToolCallCard, ToolCallGroup};